This invention relates in general to vehicle disc brake assemblies and in particular to an improved structure for a piston adapted for use in such a vehicle disc brake assembly.
Most vehicles are equipped with a brake system for retarding or stopping movement of the vehicle in a controlled manner. A typical brake system for an automobile or light truck includes a disc brake assembly for each of the front wheels and either a drum brake assembly or a disc brake assembly for each of the rear wheels. The brake assemblies are actuated by hydraulic or pneumatic pressure generated when an operator of the vehicle depresses a brake pedal. The structures of these drum brake assemblies and disc brake assemblies, as well as the actuators therefor, are well known in the art.
A typical disc brake assembly includes a rotor which is secured to the wheel of the vehicle for rotation therewith. A caliper assembly is slidably supported by pins secured to an anchor plate. The anchor plate is secured to a non-rotatable component of the vehicle, such as the vehicle frame. The caliper assembly includes a pair of brake shoes which are disposed on opposite sides of the rotor. The brake shoes are operatively connected to one or more hydraulically actuated pistons for movement between a non-braking position, wherein they are spaced apart from opposed axial sides or braking surfaces of the rotor, and a braking position, wherein they are moved into frictional engagement with the opposed braking surfaces of the rotor. When the operator of the vehicle depresses the brake pedal, the piston urges the brake shoes from the non-braking position to the braking position so as to frictionally engage the opposed braking surfaces of the rotor and thereby slow or stop the rotation of the associated wheel of the vehicle.
A considerable amount of heat is generated between the rotor and the brake shoes during braking. In a disc brake assembly having a piston constructed from a metallic material, the heat generated during braking will not usually damage the surface of the open end of the piston. Unfortunately, a disc brake piston which is formed from a metallic material is relatively expensive. It is less expensive to manufacture a disc brake piston from a plastic material than from a metallic material. U.S. Pat. No. 4,928,579 to Emmett, U.S. Pat. No. 4,449,447 to Yanagi, U.S. Pat. No. 4,401,012 to Emmett, and Japanese Patent No. 5718857 disclose prior art disc brake pistons. However, it has been found that the heat generated during braking can damage the surface of a piston formed from a plastic material. Thus, it would be desirable to provide an improved structure for a piston adapted for use in a vehicle disc brake assembly which is durable, yet relatively inexpensive to manufacture.